Certain earth orbits are preferred for the placement of satellites. For example, the orbit at 800 Km (500 st. mi.) altitude for solar-synchronous polar orbits, or the geo-synchronous orbits for weather and communication satellites at 36,000 Km (22,000 st. mi.). Normally, these satellites contain either a dual mode or single mode propulsion system including an apogee propulsion system and orbit and position control thruster systems. In the dual mode these systems are supplied by a separate propellant tank and by an oxidizer tank. In the single mode the apogee propulsion may use a solid fuel while the control thrusters are supplied by a single substance fuel tank containing, e.g. hydrazine. The propellant is usually stored in surface-tension tanks as is known, for example, from German Patent Publication DE-OS 3,146,262.
The increasing number of active and inactive satellites in these orbits and the requirement for a minimum separation between them for a safe operation leads to a foreseeable lack of suitable orbital positions. The past practice of letting the satellites simply drift away after the end of their useful life does not appear to be acceptable anymore, especially in geo-synchronous orbits which are divided into specific sectors, the utilization of which is assigned to different countries.
A suitable solution to the above described crowding is sending satellites into a higher orbit at the end of the useful life. For this purpose, only a relatively small quantity of propellant is required.
Another problem with the construction of current satellites is the difficulty to accurately determine, after about 10 years of active life, the quantity of propellant still remaining on board. To assure that the satellite can safely be removed from its orbit, it is customary to leave a relatively large remnant of propellant in the storage tanks after the orbital change has been made. This fuel could have been used for the position control and hence, it would have extended the longevity of the satellite if the true filling level of the tanks would be known.